1. Field of the Invention
The present invention relates to an optical fiber and to an optical fiber amplifier and an optical fiber laser light source both incorporating the optical fiber.
2. Description of the Background Art
An optical fiber amplifier and an optical fiber laser light source sometimes incorporate a double-cladding-type optical fiber as the amplification medium. This optical fiber is composed of (a) a core region doped with metallic ions that are excited when a pump lightwave is supplied, (b) a first cladding region surrounding the core region, and (c) a second cladding region further surrounding the first cladding region. A solid region composed of the core region and the first cladding region guides the pump lightwave, and the core region guides a signal lightwave. Because the metallic ions are excited by the pump lightwave, when a signal lightwave travels in the core region, the metallic ions emit stimulated-emission light to amplify the signal lightwave.
As the foregoing double-cladding-type optical fiber, in recent years, a holey fiber has been used that has a second cladding region provided with a plurality of holes stretching along the length of the fiber. (See, for example, a literature written by C. Simonneau et al., “High-power air-clad photonic crystal fiber cladding-pumped EDFA for WDM applications in the C-band,” ECOC-IOOC 2003, Proceedings Volume 6—Post-Deadline Papers, Sep. 21-25, 2003, pp. 34-35.) In this holey fiber, the refractive-index difference of the first cladding region to the second cladding region is further increased. As a result, the numerical aperture of the solid region is increased. Consequently, the solid region can confine an increased quantity of the pump lightwave to guide it. Thus, the excitation efficiency of the metallic ions is increased.
However, the conventional double-cladding-type optical fiber has a first cladding region whose center is nearly coincident with that of the core region. Therefore, in the case of a pump lightwave travelling in a mode having no intensity peak in the core region, the metallic ions cannot be excited. In other words, the energy of the pump lightwave cannot be used efficiently to excite the metallic ions.